Sandblasting is performed by injecting sand or other abrasive particulate material into a compressed air stream which expands to atmospheric pressure through a nozzle, hurling the abrasive material against a target at high velocity.
One common method for introducing abrasive into the air stream is to store the abrasive in a container which is pressurized to the same pressure as the air stream, and allowing the abrasive to enter the air stream under the influence of gravity, preferably through a metering device. Current OSHA regulations require that the blasting cycle, that is, on-off control of the air-abrasive stream, be controlled by the operator at the nozzle with some type of "deadman" control. In other words, the blasting function is required to be automatic.
On larger units, the pressure vessel is constantly maintained under pressure, and the sandblast air is cycled on and off either in conjunction with an abrasive metering valve or is controlled with a guillotine type valve. On smaller units, the entire pressure vessel is pressurized and de-pressurized with each sandblasting cycle, allowing continuous filling of the unit through a poppet-type valve, and requiring an automatic valve to de-pressurize the pressure vessel. This latter type of sandblasting unit is the type for which the invention is designed.
Since the pressure vessels used to contain sand or other abrasive material with this type of sandblasting unit are relatively small, when the pressure vessel is de-pressurized through a blowdown valve a great deal of abrasive is carried through the blowdown valve, causing frequent valve failure resulting from internal abrasion of the valve. Blowdown valves have been protected by the use of various filters, traps, strainers, and the like, none of which has proved satisfactorily effective, either because of design limitations resulting from the fact that at least some dust will pass through the blowdown valve with the air, or because of user neglect and/or abuse. Many types of valves have been tried in this service, with only limited success.
Since sandblast hose is especially designed to carry the abrasive or aggressive air-sand mixture, a guillotine type valve should produce the most reliable service, and should also eliminate the need for a filter or strainer for protection of the blowdown valve. For these reasons, the apparatuses provided by this invention incorporate a blowdown valve of the guillotine type. Automatic control of sandblasting requires that the air control valve which controls air introduction to the sandblast hose and the blowdown valve which controls air exhaust from the pressure vessel must be operated so that one of these valves is closed when the other is opened. A double valve, that is, a valve structure which includes two valves, one for air control and the other for blowdown control, meets this requirement. Consideration of and experimentation with different valve type combinations has resulted in the designs presented by the present invention, each of which includes a rolling diaphragm or piston actuated air valve and a guillotine type blowdown valve. The double valve apparatuses, together with their methods of operation, solve many of the problems heretofore encountered in sandblasting methods and apparatuses, and provide improved and more reliable service as compared with heretofore known methods and apparatuses.